1. Technical Field
The present invention relates to a surface-mounted piezoelectric resonator and a piezoelectric oscillator and a piezoelectric filter including such a piezoelectric resonator.
2. Related Art
Conventionally, in piezoelectric devices such as a piezoelectric resonator and a piezoelectric oscillator and a piezoelectric filter including a piezoelectric resonator, a connection pad on a piezoelectric resonator element has been jointed (secured) to an electrode pad on a package using an electrically conductive adhesive or a gold bump.
For example, JP-A-2000-332572 discloses a technique relating to a structure of a piezoelectric device which is suitable for wire bonding connection and reduces influence of the electrically conductive adhesive on various characteristics of the piezoelectric resonator.
In addition, JP-A-2000-232332 discloses a surface-mounted piezoelectric resonator that eliminates problems that arise when securing an Au bump formed on an Ag connection pad provided on a quartz crystal resonator to an Au internal terminal within a ceramic package by thermocompression bonding. Examples of such problems include: (a) excessive heating causing internal stress by heat distortion on a quartz blank that forms part of the quartz crystal resonator; (b) oxidation of an Ag layer forming the connection pad causing fluctuation of a resonance frequency; and (c) occurrence of stress inside the quartz blank due to warpage of the package.
JP-A-2000-332572 is a first example of related art.
JP-A-2000-232332 is a second example of related art.
JP-A-2005-216508 is a third example of related art.
The electrically conductive adhesive is composed of organic resin, such as silicone, epoxy, or the like, and silver particles, and curing and shrinking of such organic resin causes the piezoelectric resonator element to adhere to the electrode pad on the package, and reduction in volume of the organic resin brings the silver particles into contact with each other to ensure conductivity therebetween. Conventionally, application of the electrically conductive adhesive has generally been performed using a dispensing method, but with an increasing reduction in size and profile of piezoelectric devices, the sizes (i.e., areas) of electrode pads on packages (substrates) in which the piezoelectric resonator elements are disposed have been reduced. For example, in the case of a 2016 size (i.e., 2 mm×1.6 mm) piezoelectric resonator, the size of an electrode pad is approximately 0.30 mm.×0.30 mm Therefore, considering irregularity when applying the electrically conductive adhesive on the electrode pads, there is currently a need to reduce the size of the electrically conductive adhesive, which has been applied using a dispenser, to Φ0.20 mm or shorter.
A surface-mounted piezoelectric resonator becomes easy to be damaged by mechanical impact, if the piezoelectric resonator element becomes thin along with higher frequencies, for example. In order to absorb a shock of the impact, an electrically conductive adhesive including a silicon resin (an electrically conductive silicon adhesive) is generally used to connect a package and a piezoelectric resonator element.
The constituents of the electrically conductive silicon adhesive are generally 80 to 90 wt % silver particle, 15 to 25 wt % resin, and 5 to 15 wt % solvent. The viscosity is from 200 to 250 dPa·s and the thixotropic ratio is 3.0 to 6.0, commonly.
However, since the electrically conductive adhesive is a paste containing approximately 20 wt % resin, after application of the adhesive, it spreads and overflows out of the electrode pad. In other words, there is a problem in that it is impossible to successfully apply the adhesive so as to have a sufficiently small diameter.
FIG. 6 illustrates various electrically conductive adhesives A, B, C, and D applied on electrode pads on a package using the dispensing method. As shown in FIG. 6, each of the electrically conductive adhesives A, B, C, and D spreads after application on the electrode pads, so that the size thereof is not restricted to Φ0.20 mm or shorter.
When the adhesive is applied so as to have a diameter of 0.20 mm, a needle having an inside diameter of approximately 0.10 mm is employed, but this involves a problem in that an ejection path in the needle is clogged with the silver particles, making successive application of the adhesive impossible.
Moreover, the electrically conductive adhesive gives off an organic matter (gas) by being heated even after being cured. If the gas is condensed to defile a blank surface of the piezoelectric resonator element, a change of frequency occurs.
Particularly in recent years, when the reduction in size and profile of the piezoelectric devices has reduced the volume of a space in which the piezoelectric resonator element is contained, the piezoelectric devices have a structure that permits such gas to exert a striking influence, resulting in failure to achieve desired characteristics.
In short, the recent reduction in size and profile of the piezoelectric devices has made it difficult to accomplish adhesion of the piezoelectric resonator element by application of the electrically conductive adhesive using the dispensing method such that the applied adhesive has a sufficiently small diameter. In addition, there is a problem in that generation of the organic matter (gas) causes failure to achieve desired characteristics.
Accordingly, as disclosed in the second example of related art, a method has been proposed of achieving joining of the piezoelectric resonator element using a gold bump made of a beaten gold wire instead of the electrically conductive adhesive.
FIG. 7 illustrates an exemplary joining method for the piezoelectric resonator element using the gold bump. In the joining method as illustrated in this figure, while stage-heating is performed by a hot plate 110, a gold bump 103 is disposed to stand on an electrode pad 102 on an inner bottom surface 101 of a package 100, and pressure is exercised by a tool 111 from above upon a piezoelectric resonator element 104 with a required load, so that the piezoelectric resonator element 104 is joined to the electrode pad 102.
In this case, since the gold bump 103 is made of a relatively hard material having a high density and Young's modulus, deformation occurs at a joining point of the piezoelectric resonator element 104. As such, a heat treatment of heating the package 100 has been performed to eliminate the deformation. However, there is a problem of a considerable change in frequency and crystal impedance caused by the heat treatment, resulting in failure to achieve desired characteristics.
On the other hand, as disclosed in the third example of related art, a method of using a metal powder as a metal bump to form a soft bump has been proposed.
However, the metal bump disclosed in the third example of related art joins joining portions after drying and sintering a metal paste to form a metal bump.
When the piezoelectric resonator element 104 that is not to be joined is pressed to the metal bump described the above, the metal bump is crushed and has a structure equal to the gold bump disclosed in the second example of related art.
Therefore, even when the metal bump disclosed in the third example of related art is used, deformation at a joining point of the piezoelectric resonator element 104 may be unavoidable in some cases because the density and Young's modulus of the metal bump are high after the piezoelectric resonator element is mounted.
Further, the metal bump disclosed in the third example of related art is made by dispersing a solvent to dry before the joining portions are joined.
Therefore, even if an end part of the piezoelectric resonator element 104 is mounted on the metal paste so that the piezoelectric resonator element 104 is secured on the metal bump by being held at one side, the metal paste cannot serve sufficient adhesive strength (wettablity) to hold the piezoelectric resonator element 104 in a horizontal position.
Accordingly, before the metal paste is sintered, the free side of the piezoelectric resonator element 104 inclines toward the bottom of the package to contact. This process may give a quartz crystal resonator insufficient resonating characteristics.